Conventional rubber cup seals employed in master brake cylinders are prone to wear and distortion, thus requiring replacement thereof after extended use. The cup seal is normally reciprocated in a working bore by a push-rod assembly to force hydraulic fluid into the wheel cylinders of band or disc brakes. The brake cylinder has a fluid reservoir which communicates with the bore via refill and compensating passages to continuously supply adequate fluid thereto for brake actuation purposes.
Upon movement of the cup seal into masking relationship over the passages, portions of the seal will tend to become distorted thereat. Such distortion is primarily occasioned by the high fluid pressures, e.g. 1,800 psi, prevalent in the bore of the cylinder during brake actuation. In particular, portions of the seal tend to be extruded into the above mentioned passages to eventually permanently damage and/or distort the seal to render the braking system leak prone and possibly inoperative.